1. Field of the invention
This invention relates to an electrochromic display device and to methods for producing same, and more particularly to electrochromic display devices, methods, and compositions for electrochromic devices with greatly enhanced color rendition.
2. Description of the Prior Art
Laminate electrochromic displays are known, such as those described in U.S. Pat. No. 5,413,739, the disclosure of which is incorporated in its entirety by reference. Some known techniques of fabricating electrochromic displays involve side-by-side electrodes, such as illustrated in FIG. 1, wherein a nonconductive substrate A is coated with a conductive layer B of a plurality of electrodes such as metal, metal oxide, conductive polymer, or carbon. An additional layer C.sub.1 is coated on top layers B and A, and comprises an electrically conductive, essentially ionically isolative, electrochromic composite layer comprising a dispersion of electrically conductive, electrochromic particles dispersed in a polymer matrix, e.g., titanium dioxide particles coated with ATO (antimony-doped tin oxide) dispersed in a rubber matrix. Layer D is an ionically conductive layer, e.g., an aqueous polymeric gel such as polyacrylamidomethylpropanesulfonate (known as POLYAMPS) gel, while layer F is a transparent, insulating layer such as a polyethylene terephthalate (PET) film, that serves to prevent loss of electrolyte from the conductive layer. Because the conductivity of the electrochromic composite layer C.sub.1 is lower than the conductivity of the ionically conductive layer D, current will preferentially flow from one electrode through the electrochromic layer to the second electrode. Where the electrochromic material changes color with the loss of an anion, the electrochromic effect will be visible over one electrode. Where the electrochromic material changes color with both the gain and loss of an anion, e.g., as is the case with polyaniline, electrochromic effects will be visible over both electrodes.
Doped tin oxides are known among metal oxides for their relative transparency and high electrical conductivity. These properties are advantageous employed in a variety of electro-optical applications, e.g., providing transparent conductive coatings on particles or surfaces. One such application is the fabrication of transparent electrodes on electrochromic display devices which typically have an electrolyte material in contact with an electrochromic material so that an electrochromic effect is generated when an electric potential is applied across the interface of the two materials. When electrodes are provided on both sides of the materials, e.g., in sandwich-like structure, the electrode on at least one side of the display laminate needs to be relatively transparent to permit observation of the electrochromic effect. In such devices, typical electrochromic materials include tungsten oxide, Prussian blue, polyaniline and viologens. Transparent electrodes have been fabricated by vapor deposition of doped tin oxide coatings on glass or plastic substrates. Although doped tin oxides have been employed as transparent conductors in electrochromic devices, it appears that the possibility that doped tin oxide might be useful as a practical electrochromic material has not been discovered. For example, Orel et al. reported in the Journal of the Electrochemical Society, Vol. 141, page L127 (1994) that a film of ATO exhibited a change in light reflectance between the reduced and oxidized state of less than 5% which corresponds to a contrast ratio (as defined hereinbelow) of less than 1.05. Because such a change in color is not readily discernable to the typical human eye, it has not been recognized or discovered that doped tin oxides have useful electrochromic properties.
A variety of dopants are used to make conductive metal oxides, some of which are not known to exhibit any useful electrochromic effect regardless of modification. Similarly, ATO, when provided in film form, also does not exhibit any useful electrochromic effect.
Pigments used in electrochromic displays such as those described above, as well as others having different electrode arrangements, such as sandwich-type displays, wherein electrochromic material is sandwiched between two electrodes, one of which is transparent, have historically had a somewhat limited range of colors. When it has been desired to add color to previous sandwich-type electrochromic displays, an opaque TiO.sub.2 -containing electrolyte with added color pigments has been used. However, this method is not suitable with electrochromic displays having interdigitated (or side-by-side) electrodes, because the electrolyte must be transparent in order to permit the color-changing substances behind it to be seen. Therefore, other methods must be used.